1. Field of the Invention
The present invention relates to a coaxial type starter motor which is connectable to an electromagnetic switch at its rear portion and is provided with an output shaft with a pinion at the front end portion of the output shaft.
2. Discussion of Background
FIG. 2 is a front view partly cross-sectioned in the longitudinal direction of a conventional starter motor disclosed in, for instance, Japanese Unexamined Patent Publication No. 149349/1991 wherein the upper half portion of the output shaft of the starter motor indicates a retracted, returned state and the lower half portion indicates an advanced state. In FIG. 2, numeral 1 designates a d.c. motor having an armature in which a hollow rotary shaft 2 is disposed. The front portion of the rotary shaft 2 is supported by a bearing supporting portion 17a formed in a yoke 17 by means of a bearing 18. Numeral 4 designates an output shaft the rear end of which is extended into the rotary shaft 2 and is supported by a bearing 14. The front end of the output shaft 4 is connected with a pinion 3 through a spline connection, and a stopper 12 is disposed at the front end of the pinion 3. Numeral 8 designates a front bracket of the d.c. motor 1. A planetary reduction gear device 7 comprises a sun gear formed at the front end of the rotary shaft 2, an internal gear wheel 7b fixed inside the front bracket 8, and a plurality of planetary gears 7d interlocked with the sun gear 7a. Each of the planetary gears 7d is supported by a supporting pin 7c. The planetary gears 7d are also interlocked with the internal gear wheel 7b to effect a revolving motion. A carrier 19 is supported by the rotary shaft 2 by means of a bearing 20, and the supporting pins 7c are firmly attached to the carrier 19.
An overrunning clutch 9 transmits revolution reduced by the planetary reduction gear device 7 to the output shaft 4 in one direction. The overrunning clutch 9 has a construction as follows. A clutch outer 9a is firmly attached to the carrier 19 and a clutch inner 9b is fitted to the clutch outer 9a by means of rollers 9c to transmit a one way revolution force to the output shaft 4. Numeral 21 designates a fitting plate, and numeral 22 designates a cover fixed by caulking to the clutch outer 9a and connected to the clutch inner 9b in the axial direction. The cover 22 seals grease charged therein. The clutch inner 9b is supported by the front bracket 8 by means of a bearing 11.
The output shaft 4 has a helical spline portion 4a which is interlocked with a helical spline groove 9d formed in the inner circumference of the clutch inner 9b to thereby transmit a rotation force.
A cylindrical portion 15 is extended from the rear end of the pinion 3, and the outer periphery of the cylindrical portion 15 is slidably supported by the inner periphery of the clutch inner 9b. On the other hand, the inner periphery of the cylindrical portion 15 supports the outer periphery of the front portion of the output shaft 4 so as to be slidable. Numeral 10 designates a return spring to retract the output shaft 4, and numeral 13 designates a shock absorbing spring which pushes the pinion 3 forwardly.
Numeral 16 designates an electromagnetic switch attached to the rear portion of the d.c. motor in a coaxial manner. The electromagnetic switch 16 is provided with a movable rod firmly attached with a movable iron core. The front end of the movable rod pushes a push rod by means of a compression spring. The front end of the push rod faces the rear end of the output shaft 4 by means of a steel ball (each not shown). The electromagnetic switch 16 is so constructed that the movable iron core is attracted to the fixed iron core electromagnetically so that a movable contact supported by the movable rod is brought into contact with a fixed contact, whereby current is fed to the d.c. motor.
In the conventional device described above, when a starter switch is turned on, the output shaft 4 is moved forwardly by the action of the electromagnetic switch 16 through the push rod, and the pinion 3 attached to the output shaft 4 is meshed with the ring gear of the engine. Then, the movable contact is brought into contact with the fixed contact so that the main circuit of the d.c. motor 1 becomes conductive, and the rotary shaft 2 is rotated by the armature. The rotational speed of the rotary shaft 2 is reduced by the planetary reduction gear device 7, and the reduced revolution is transmitted to the output shaft 4 through the overrunning clutch 9, whereby the revolution is transmitted to the engine by means of the pinion 3. When the engine is started and the starter switch is turned off, the output shaft 4 is retracted by means of the return spring 10, and the movable rod, movable iron core and movable contact are retracted and returned by means of the pushing rod.
In the conventional coaxial type starter motor, the axial center of the clutch outer 9a is determined by the carrier 19, the rotary shaft 2, the bearing 20, the bearing supporting portion 17a and the bearing 18. On the other hand, the axial center of the clutch inner 9b is determined by the inner periphery of the front bracket 8 to which the bearing 11 is fitted. The front bracket 8 is fixed to the yoke 17. Since a number of elements are fitted to each other, it is difficult to maintain the clutch outer 9a and clutch inner 9b in a coaxial manner. Especially, when the engine is an overrunning state, deflection of the axial centers of the clutch outer 9a and the clutch inner 9b easily takes place. As a result, durability of the overrunning clutch 9 may be reduced.